Process for the continuous removal of undesirable components of solid particles by a solvent applied in counterflow

ABSTRACT

A cyclical and automatable process and apparatus for continuously removing undesired components from solid particles by means of a solvent, the process including maintaining an expanded bed of solid particles in a vertical column, supplying a suitable solvent to the bed to flow through the bed either upwardly or downwardly in a selectably alternating manner, supplying solid particles to the top of the column while the solvent flows upwardly therethrough and interrupting the supply of solid particles and removing a bottom portion of the bed by the solvent when it flows downwardly.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of applicationSer. No. 7,375, filed Jan. 29, 1979, now U.S. Pat. No. 4,304,742.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a process for the continuous removal ofundesirable components from solid particles, and is particularlydirected to for the removal of soluble components from microspherematerial to be used for the production of nuclear fuel, the solublecomponents being removed using a solvent streaming through a bed of themicrosphere material in counterflow. The invention also is directed toan apparatus for performing the process.

2. Description of the Prior Art

In nuclear reactor technology a particulate material containing thorium,uranium or plutonium is used in the manufacture of nuclear fuel. In thisregard, fuel elements used in gas-cooled high-temperature reactors areformed of a graphite matrix in which fission or breeder materials in theform of coated microspheres are embedded. Also, the loading of fuel rodsused in light water and breeder reactors can be carried out with aparticle-shaped fuel (Spherepac-Fuel). The diameter of such particles isin general in the range between 50 μm and 5 mm.

As a rule, such microspheres are produced by wet-chemical methods. Inthese processes the microspheres are cast, whereby a solution ofsuitable composition separates into drops and undergoes a chemicalprocess resulting in the solidification of the drops. The solidifieddrops (microspheres) are as a rule of a gel-like consistency and thusnot very resistant to mechanical stresses.

After solidification the microspheres will generally still containby-products which have to be eliminated from the microspheres beforethey are further processed. This is achieved by dissolving theundesirable components in a suitable solvent.

The microspheres used for the production of nuclear fuel contain toxic,radioactive components, a part of which is carried over into the solventin the course of the dissolution of undesirable components. The usedsolvent, however, can only be permitted to be released into theenvironment after special conditioning treatment and is therefore mostlyrecycled. Consequently, for economical and ecological reasons, solventsshould be used as sparingly as possible, i.e., the solvent should becharged with foreign bodies to the maximum possible extent. For thetreatment of such particles, systems involving continuous counterfloware used.

In known apparatuses for the continuous removal of soluble components ofsolid particles, the solid substance is carried in counterflow in astream of solvent either by a device in the fill of solid material or bya conveying device for the solid material.

To the first group belong equipment using a screw conveyor (HildebrandtExtractor). In such equipment, since the product of extraction isexposed to mechanical stresses caused by the movement of the screw, thepurification of microspheres in such devices can cause damage to or evendestruction of the product.

To the second group of equipment belong extractors fitted, among others,with a conveyor belt, a bucket elevator and a bucket-wheel. Theseextractors are mechanically complicated and are, as a rule, unsuitablefor products that are too delicate for mechanical handling. Furthermore,in handling radioactive materials there are additionally decontaminationproblems which, for this kind of apparatus, are difficult to solve andare accordingly expensive.

In other known processes a solvent is forced through a solid bed made upof the particles to be purified. Experiments have shown that thepressure drop across the solid bed can also damage the mechanicallydelicate microspheres. In addition, as is known, in the case of solidbeds there is a risk of channel-forming. In such cases, the solvent nolonger flows uniformly through the pile or fill of microspheres butrather it flows through the individual channels in the bed ofmicrospheres so that the particles are not sufficiently purified.

The object of the present invention is therefore to eliminate or reducethe drawbacks of the known processes and to provide a process andapparatus which can vouchsafe that undesirable components aresubstantially eliminated from mechanically delicate solid particleswhile maintaining their shape.

SUMMARY OF THE INVENTION

According to the invention the process for the continuous removal ofundesirable components from solid particles is characterised by thecyclical sequence of the following process steps:

(a) loading the solid particles into a static column of solvent throughan upper decanter zone which, e.g. is widened relative to the column;

(b) sustaining an expanded bed of solid particles in the column by astream of solvent flowing at a predetermined velocity introduced intothe column at its base and removed from the decanter zone, whereby theinflowing solvent is pure and the outflowing solvent is charged withundesirable components washed out from the solid particles;

(c) interruption of the charging of solid particles and the supply ofsolvent at the upper and at the lower end of the column of solvent,respectively, opening the solvent column at the lower end and supplyingsolvent to the upper end of the column, unloading a lower section of thebed of solid particles in the column via its open lower end by a streamof solvent directed downwardly in the column, and

(d) interrupting the discharge of solid particles at the lower columnend and the feeding of solvent to the upper end of the column andopening of the solvent column both at the upper end for feeding in acharge of solid particles and at the lower end for feeding in solvent.

For the purpose of automation of the process, the concentration of thecomponents dissolved in the solvent can be measured at at least oneposition of the column between the upper and the lower solvent feedinlets, preferably by determination of the electrical conductivity ofthe solvent and a changeover of the feeding of the solvent from thelower end to the upper end of the solvent column is controlled accordingto the measured values of conductivity. The supply of the solvent intothe column at its lower end can be carried out during a firstpredetermined time period, and the downwards movement of the bed ofsolid particles for the discharge by means of the feeding-in of solventinto the column at its upper end can be carried out during a secondpredetermined time period so that the individual process steps areautomatically repeated in each process cycle and in any discretionallyselected number of cycles.

According to the invention, apparatus suitable for carrying out theprocess is characterised in that it consists of a vertical tube fittedat each of its ends with a respective inlet for feeding in a solvent andwith a respectively essentially coaxial valve for charging in anddischarge of solid particles; an essentially coaxial decanter vesselwith a filling hole for solid particles arranged over the upper inletvalve for charging in the solid particles and with an overflow for thesolvent, a discharge device arranged under the lower outlet valve forthe discharge of solid particles and a valve arrangement connected tothe solvent inlets for the alternate supplying of solvent into thecolumn through one or the other inlet of the tube.

At least one measuring device for the determination of the concentrationof the undesirable components of the solid particles can be arrangedbetween the two inlets of the tube whereby expediently a device formeasuring of the electrical conductivity of the solvent is used. For thepurpose of automation of the device, the apparatus can be provided witha control unit which, in accordance with the signals of the measuredvalues obtained from the measuring device controls the valve arrangementfor the changeover of the supply of the solvent into the standpipe and,according to a planned time schedule controls the duration of thecharging of the solid particles via the inlet valve and the duration ofthe discharge of the solid particles via the outlet valve.

One application of the process is the preparation of nuclear fuel. Thenuclear fuel containing thorium, uranium or plutonium is passed into thesolvent column in the form of solid particles and the column is filledwith a solvent which dissolves the undesirable soluble components of theparticles of nuclear fuel. The process is particularly advantageous whenthe nuclear fuel particles consist of wet-chemically producedmicrospheres.

BRIEF DESCRIPTION OF THE DRAWING

The invention is further explained in detail purely by way of example inthe accompanying drawing, the single FIGURE of which is a diagrammaticalillustration of a preferred embodiment of apparatus for performing theprocess according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As can be seen from the drawing, the illustrated apparatus for thecontinuous elimination of undesirable components from solid particles byuse of a solvent comprises an extraction-column which forms of adecanter zone 1, a charging zone 2 and a discharging zone 3.

The extraction column consists of , e.g., a standpipe or vertical tube2' forming the charging zone 2, a decanter vessel 1' located at theupper end of the tube 2' and which is, relative to the tube 2',essentially coaxial therewith (the vessel 1' including a larger orwidened section and which constitutes the decanter zone 1), and adischarge device 3' (constituting the discharging zone 3) located at thelower end of the tube 2'.

The direction of flow of the solvent stream during the charging phase isindicated by an arrow A and the direction of the flow of the solidparticles is indicated by an arrow B shown by a broken line.

The solid particles may, e.g. be microspheres obtained from theso-called Spherepac process of nuclear fuel manufacture. In this case,the by-product to be extracted is, e.g., ammonium nitrate and othersoluble components. The solvent is in this case water or an aqueoussolution of ammonia.

When operating the process certain conditions have to be observed andcontrolled, such as the composition of the liquid in the differentzones, so that no insufficiently leached particles can pass out from thecolumn.

The decanter vessel 1' has an overflow port 4 for the solvent. The solidparticles are charged through an upper filling aperture 5 into thedecanter vessel 1' and settle at the bottom of the decanter zone 1.

The charging zone 2 in the form of a preferably vertical tube 2' isbounded at the top by a particle inlet valve 6 and at the bottom by aparticle discharge valve 7. A first, e.g. lateral, inlet 8 for thesolvent is located at the lower end of the charging zone 2, above theparticle discharge valve 7. The charging zone 2 is furthermore providedwith a test or measuring position 14 for measuring changes in theconductivity of the solvent. At the upper end of the charging zone 2,there is a second inlet 9 for the solvent.

The first and second solvent inlets 8, 9 are connected to solvent supplyline 13 via a respective solvent inflow control valves 10 and 11, eachvalve being controlled by a control unit 12. The supply line 13 isprovided with respective regulating (e.g. needle) valves 15 and 16 forthe separate or independent setting of the flow rate of solvent frombelow upwardly or from the top downwardly.

The discharging zone 3 has a device, e.g., a syphon, for preventing airfrom penetrating into the column while posing the least possiblehindrance to discharging the washed or leached particles.

The measuring location 14 is connected to the control device 12 thatcontrols the action of the valves 6, 7, 10 and 11.

When the column is in operation during the charging cycle, the particleinlet valve 6 and the lower solvent inlet valve 10 are open. The solventor the flushing liquid streams through the column from the bottomupwardly and exits from the column at the overflow 4. Microspheresdisposed in the decanter zone trickle into the charging zone until thelatter is filled up. The gap ratio in the microsphere bed (ratio oftotal column less the volume of the microspheres to the total volume)can be set by the rate of flow of the solvent or flushing liquid at thelower regulating valve 15. To avoid formation of channels in the bed,particularly in the case of very fine particles (diameter of less than1000 μm), the velocity of flow is set in such a way that an expanded bedis obtained in relation to a solid bed. The very fine particlesentrained by the stream of the solvent or flushing/washing liquid areretained in the decanter zone due to the locally reduced velocity of thestream in the decanter zone that is widened relative to the chargingzone.

If the conductivity of the flushing liquid at the measuring point 14falls below a predetermined value, i.e., the concentration of theundesirable components in the microspheres between the discharge valve 7and the measuring point 14 falls below a certain value, the particlecharging valve 6 and the lower solvent inlet valve 10 are automaticallyclosed and the particle discharge valve 7 and the upper solvent inletvalve 11 are opened. The stream of the solvent or flushing liquid causesthe microsphere bed to be shifted downwardly by a certain distance withthe liquid now flowing from the top downwardly towards the bottom; inthis way, a portion of the washed microspheres is conveyed through thedischarge zone 3. The length of the distance by which the bed is moveddownwardly is determined on the one hand, by the magnitude of thedownwardly flowing stream of liquid and on the other hand, by theduration of the time period during which the valves 7 and 11 remainopen. After expiration of a stipulated time period, the particledischarge valve 7 and the upper solvent inlet valve 11 are closed andthe particle charging valve 6 and the lower solvent inlet valve 10 areopened. At this stage, the unwashed microspheres from the decanter zone1 trickle into the part of the charging zone 2 that was emptied duringthe discharging cycle. The operating sequence of the cycle issubsequently repeated, as described above.

The advantages of the described arrangement are that it has a simplermechanical construction, hence it is suitable also for radioactivematerials; it provides a gentle, careful handling of the microspheressince no mechanical transportation of the microspheres is necessary; noformation of channels is caused and only a small pressure drop acrossthe bed since the gap ratio of the bed exists is automatically adaptedto the velocity of flow of the flushing liquid. In many cases, theefficiency of the washing out of the microspheres may be furtherimproved if the ascending stream of solvent is caused regularly orirregularly to pulsate to which end e.g., the open lower solvent inletvalve 10 is suitably set, preferably automatically.

EXAMPLE

    ______________________________________                                        Height of column:        1400 mm                                              Diameter of charging zone:                                                                             15 mm                                                Height of charging zone: 800 mm                                               Diameter of microspheres:                                                                              300 μm                                            Solvent flow rate:       1 l/h                                                Duration of the charging cycle:                                                                        2 min                                                Duration of the discharge cycle:                                                                       8 sec                                                Throughput of microspheres:                                                                            300 g/h                                              Efficiency of washing-out of the                                                                       95%                                                  microspheres:                                                                 ______________________________________                                    

I claim as my invention:
 1. A process for the removal of undesirablesoluble contaminants from solid particles by passing a solvent for thecontaminants through a bed of the contaminated solid particles, theprocess comprising the following sequential steps:(1) charging thecontaminated solid particles into the upper end of a column of solventby way of an upper decanter zone which has a greater diameter than thatof the solvent column; (2) adding solvent to the solvent column near itslower end and causing the solvent to move at a predetermined rateupwardly, thereby causing the contaminated solid particles in thesolvent column to form an expanded bed, (3) removing solvent withdissolved contaminants therein from the upper decanter zone, (4) after afirst predetermined time period has passed, stopping the charging ofcontaminated solid particles to the top of the solvent column and theadding of solvent to the base of the solvent column, (5) adding solventto the solvent column near its upper end and concurrently removing asection of the bed of solid particles from the lower end of the solventcolumn for a second predetermined time period, (6) at the conclusion ofsaid second predetermined time period, stopping both the supply ofsolvent to the upper end of the solvent column and the removal of solidparticles from the lower end of the solvent column, and (7) repeatingsteps (1)-(6).
 2. A process according to claim 1, wherein said solidparticles are particles containing thorium, uranium or plutonium.
 3. Amethod according to claim 1, wherein said solid particles are gel-like.4. A method according to claim 1 wherein said solid particles are in theform of microspheres produced by a wet-chemical process.